Electric power steering apparatuses are conventionally known which are adapted to provide a steering power assist by energizing an electric motor in accordance with manual operation of a steering wheel during travel of a motor vehicle. In the electric power steering apparatuses, a steering torque signal from a steering torque detector that detects a steering torque on a steering shaft produced when the steering wheel is turned by the driver and a vehicle velocity signal from a vehicle velocity detector that detects a vehicle velocity are used to control operation of the electric motor under the control of a motor controller so as to generate a steering power assist to thereby reduce necessary steering power to be applied by the driver. In operation, the motor controller sets a target current value to be supplied to the electric motor on the basis of the steering torque signal and vehicle velocity signal, determines a difference between a target current signal indicative of the target current value and a motor current signal outputted from a motor current detector that detects a motor current actually flowing through the motor, and performs a compensating operation using a proportional plus integral control action (PI control) to thereby generate a drive control signal for controlling driving operation of the motor.
In an actual arrangement of the electric power steering apparatus, the motor is disposed near the steering shaft to ensure transmission of a rotational force from the motor to the steering shaft, and a circuit board having electronic circuits formed thereon to form the motor controller is associated with the motor. The electronic circuits of the motor controller are disposed inside an engine room with bad noise environment. Furthermore, various mechanical or electric switch elements that form a source of high frequency noises due to their on-off operation are disposed near the electronic circuits. With this arrangement, a PI control circuit, for example, of the motor controller is highly liable to be influenced by the high frequency noises, and the motor current signal that is detected by the motor current detector and is fed back to the motor controller is likely to entrain an external high frequency noise. When a high frequency noise is mixed in or superposed on the motor current signal as an external disturbance, such external disturbance component (high frequency noise) will alter the rotational force of the motor, thus making the steering power assist unstable. Unstable steering power assist provides a deteriorated steering feeling.
To deal with this problem, the present assignee has proposed a solution which, as disclosed in Japanese Patent No. 2959957, includes an attenuation device or attenuator provided in a signal path connecting a motor current detector and a motor controller, so as to attenuate or reduce a high frequency noise. More specifically, the attenuator is disposed between the motor current detector and a PI control section of the motor controller, and the filtering characteristic (frequency characteristic) of the attenuator is determined such that current noises of frequencies sufficiently higher than those of usual motor currents occurring during PWM (pulse-width modulation) control operation of the motor controller can be attenuated.
On the other hand, in recent years a brushless motor has been used in the electric power steering apparatus as a motor for providing a steering power assist. In one example shown in Japanese Patent Laid-open Publication No. 2001-106099, a three-phase brushless motor is used. In an electric power steering apparatus equipped with such three-phase brushless motor, if an attempt is made to avoid the occurrence of the aforementioned problem caused due to mixing of an external disturbance (high frequency noise) in the motor current, a similar attenuator must be provided in each of three signal paths interconnecting the respective motor current detectors and corresponding PI control sections of the motor controller that are provided in pairs for the respective phases of the three-phase brushless motor.
In the case where a three-phase brushless motor is used as a steering power assist motor of the electric power steering apparatus in such a manner as shown in Japanese Patent Laid-open Publication No. 2001-106099, a deviation of the detected motor current from the target motor current must be determined for each of the U-phase, V-phase and W-phase of the brushless motor for the purpose of performing the PI control. Thus, when a high frequency noise is to be reduced, three attenuation devices or attenuators must be provided one for each of the U-, V- and W-phase. If these attenuators are provided by way of hardware, a difficulty may arise that due to an increased number of parts, the motor controller becomes large in size and is uneasy to find a place for installation thereof within an internal space of the engine room. Alternatively, if the three attenuators are realized by way of software, a lengthy computer program adapted to be processed by a CPU (central processing unit) of the controller must be provided. In the latter case, if a low performance CPU is used, it requires a relatively long time to carry out arithmetic and logic operations. Thus, the responsiveness of the electric power steering apparatus is relatively low and the steering feeling is deteriorated. Conversely, if a high performance CPU is used, the cost of the controller increases.
In an electric power steering apparatus equipped with a three-phase brushless motor, a three-phase alternating current is used as a motor current to be supplied to the brushless motor for driving the same. In order to control the alternating motor current, the effective value of the alternating motor current must be controlled in accordance with the amount of steering power assist, and at the same time, the frequency of the alternating motor current must be controlled in accordance with the steering speed. If an attenuation device or attenuator for reducing high frequency noises is provided in a feedback loop of the motor current signal extending from a motor current detector and a PI control section of the motor controller, the attenuator will be designed to have a frequency characteristic which is capable of removing high frequency components related to the high frequency noises. In this instance, however, a risk may occur depending on the frequency characteristic of the attenuator that the alternating motor current required to generate steering assist power is attenuated or decreased by the attenuator. Especially when the steering wheel is quickly turned lock to lock in several times (high-speed steering operation), the frequency of the alternating motor current may rapidly increase to such an extent that the frequency of a motor current signal detected by the motor current detector falls in a range of frequencies of the high frequency noises. In this instance, due to the frequency characteristic of the attenuator, the motor current signal is attenuated at the same time high frequency components (high frequency noises) are attenuated. With this attenuation of the motor current, the steering assist power is caused to vary and the steering feeling is deteriorated.
Especially in case where the frequencies of the alternating motor current that is used for driving the brushless motor to provide a steering power assist are included in a frequency range of the high frequency noises, it is substantially impossible to attenuate the high frequency noises alone because the high frequency noises are mixed in the motor current signal. Thus a smooth steering feeling is difficult to obtain.
The attenuator shown in Japanese Patent No. 2959957 is used in combination with a conventional dc motor having brushes and not with a brushless motor. Furthermore, in this Japanese Patent, use of the attenuator is not based on the assumption that a range of frequencies of the high frequency noises may contain frequencies of the motor current. It is therefore logically impossible to use the attenuator of the Japanese Patent in combination with a brushless motor without a teaching of the present invention.